The present invention relates to a process of fabricating light-focusing plastic optical elements, and in particular to a process of fabricating a light-focusing plastic rod (LFPR) having a large diameter and superior imaging properties.
Light-focusing rods (LFR) are a type of gradient index (GRIN) optical element. The refractive-index distribution of a LFR is parabolic from its central axis to the periphery. Because of such property, the incident light will travel in a zigzag way through the rod and focus at the outer space behind its exiting end. This fact makes the LFR act as a concave lens and thus it is commonly called a "light focusing rod".
Among optical elements, there are three types which have the above-mentioned parabolic refractive index distribution and are cylindrical in shape. They are GI type optical fiber, GRIN rod, and Wood lens. Their image-transporting principles are the same, but lengths and application categories differ. Among them, GI type optical fiber (e.g. silica series fibers) is the longest one, usually longer than several hundred meters or even up to tens of kilometers, and is used as a transmitting medium in optical communication systems. GRIN rod has a medium length and can be used as an element of a image-transport system, optical fiber connection device, and detective sensors. In general, those having a length longer than the internal diameter but not being used as optical fiber are classfied as GRIN rod. Whereas Wood lens is in the form of a thin lens with a length shorter than its internal diameter and is used mainly in image-transport and light focusing.
There are up to now many reports and patents concerning the fabrication of LFPR. For example, U.S. Pat. No. 3,718,383 (1973), No. 3,861,160 (1974), and No. 4,581,252 (1986) disclose a non-solvent swelling diffusion method, which method comprises diffusing a variety of monomers to a plastic rod by using a mixture of non-solvent and solvent, and polymerizing these monomers to cause a specific refractive-index distribution. Such method takes advantage of the diffusion gradient of the monomers and thus produces a parabolic distribution of the refractive index. Furthermore, Ohtsuka discloses a UV-induced copolymerization method wherein the supplementation of the monomer solution is controlled to avoid bubble formation caused by the shrinkage of the polymerization [Y. Ohtsuka, Appl. Opt., Vol.24, No.24 (1985), 4316]. In addition, Ohtsuka's group also invented a two-step copolymerization process to fabricate light-focusing plastic rods. For example, the vapor-phase transfer process [Y. Ohtsuka, Appl. Opt., Vol 22, No. 3 (1983), 413], comprises preparing a partially polymerized gel rod, placing the gel rod in the atmosphere of trifluoroethyl methacrylate vapor, followed by heat treatment to yield the GRIN rod. The process disclosed in J. Appl. Polym. Sci., (Vol. 26, (1981), 2907-2915) comprises immersing a partially polymerized gel rod in 2,2,3,3,-tetrafluoropropyl methacrylate, followed by heat treatment under nitrogen. The process disclosed in Appl. Opt., (Vol. 20, No. 13, (1981), 2319) is also a two-step copolymerization process, which comprises polymerizing diethylene glycol bisallyl carbonate to form a partially polymerized gel rod, then immersing the gel rod in fluoroethyl methacrylate under nitrogen. All of the light-focusing plastic rods obtained by the two-step copolymerization process invented by Ohtsuka et al have a crosslinked structure and therefore can not be further fabricated into optical fibers. An interfacial-gel copolymerization technique is also reported by Ohtsuka (Appl. Opt. Vol. 27, No. 3, 486). By this technique, a copolymer phase gradually forms from the gel phase formed at the interface between the monomer and polymer substrate, and a gradient-index profile is thus obtained from the gel phase formed on the wall of the polymer substrate. Furthermore, U.S. Pat. No. 4,022,855 (1977) and JP 60-119510 (1985) disclose a centrifugal casting method to fabricate preform of plastic optical fibers.
All of the above-mentioned processes take different approaches to fabricate LFR having a parabolic refractive-index distribution and avoid shrinkage of the rod. However, they all need complicated procedures and equipments. For example, in the UV-induced copolymerization method, apparatus capable of producing UV light is required. Besides, in order to avoid the shrinkage of the fabricated LFR, the diameter of the plastic rod produced by the above-mentioned processes is limited and thus the portion that can be used for image transfer is relative small and sometimes will causes distortion of the image transfered.